Secondhand smoke exposure (SHSe) is a significant health risk, especially to young children. More than 50 percent of children's SHSe occurs at home and children in low-income households are more likely to be exposed to SHS. Thus, home-based interventions are needed to protect children from SHSe. This is the first efficacy study to test whether children's home SHSe can be reduced by using innovative SHS particle monitors to provide real-time feedback to household residents plus brief advice/praise. This study will test the efficacy of immediate feedback and brief advice/praise on low-income families' ability to reduce children's exposure to nicotine, as measured by cotinine assays, reduce smoking in the home, and establish home smoking bans. Learning theory and pilot data suggest that real-time feedback from the monitor plus brief advice will prompt and reinforce SHSe reduction behaviors. Four hundred families from Women, Infants, and Children (WIC) programs in San Diego, California will be recruited. Households will be randomly assigned to an experimental group that receives continuous real-time feedback from a SHS particle monitor plus feedback-related brief advice/praise to reduce their child's exposure, or to a usual care control group where the particle monitor delivers no feedback. During the intervention phase, monitors in households of the experimental group will produce warning lights and beeps when SHS is detected. The primary specific aims are to: 1) Determine the differential group change in SHSe levels, as measured by child cotinine assays, due to real-time monitor feedback and brief advice; 2) Determine the differential group change in the establishment of home smoking bans; and 3) Validate differential change in particle measures of SHSe using nicotine dosimeters and child urine cotinine assays. The exploratory aims are to: determine theoretical moderators (e.g., child's health) of the effect of brief advice plus feedback; assess the effects of theoretical mediators (e.g., establishment of home smoking bans and other social contingencies) of change in SHSe; explore the dynamic nature of SHSe by examining the continuous particle monitor records over months to identify social contexts and micro- environments in the home as precise targets for intervention (e.g., smoking location, nonsmoker locations, and social interactions while smoking); assess the differential changes in tobacco use and SHSe by racial/ethnic group and by single v two-parent families to increase understanding of disparities for tobacco control; and obtain suggestions from families to enhance future use of the monitors. This is also the first large-scale test of a new technology in residential tobacco control, where real-time and telemetry feedback to investigators will both enable immediate feedback from light and sound alerts to families and inform investigators of smoking events or reduction in smoking events that warrant advice or praise. Thus, this application opens a new direction for future innovative studies using novel technology to advance reduction of SHSe in a range of micro-environments.